Most modern robotic systems are fast and repeatable position controlled machines. However, despite extensive R&D efforts, they mostly remain confined to controlled areas where they execute specific pre-programmed actions. Furthermore, they still display limited performances in tasks such as grinding, polishing, surface following and complex assembly. Moreover, even if many economically interesting man-machine interaction applications have been identified (physical therapy, training assistance, surgery assistance, manual tasks teaching, sport training, ortheses and prostheses motorization, haptics, teleoperation of interacting machines, etc.), very few have been implemented successfully.
Over the last 25 years, some researchers tried to identify and revise design paradigms with one objective in mind: to create robotic systems capable of safe and versatile interactions, which led to the development of interaction control theory. Unfortunately, classic actuators proved to be unfit for its usage and unfit for safe and versatile interaction, primarily because of high output impedance (inertia and friction) and because of the usual non-collocation of sensing and actuating transducers when force feedback is implemented.
A safe and versatile actuator, fit for a variety of interaction tasks, should possess at least four basic characteristics: 1) high force or torque density; 2) sufficient force bandwidth; 3) very low output impedance; and 4) high-fidelity force display capability. However, no classic actuator simultaneously exhibits all of these basic characteristics.